drop un-maintained sheepdog plugin

drop un-maintained sheepdog plugin

as already announced over two months ago[0], remove the unofficial
SheepDog plugin now completely. Besides that it was never fully
supported in Proxmox VE one of its main developer and ex-maintainer
declared it as abandoned[1], and thus just let's remove it, git
allows to resurrect it any time if a wonder happens anyway.

[0]: https://pve.proxmox.com/pipermail/pve-user/2019-March/170497.html
[1]: http://lists.wpkg.org/pipermail/sheepdog/2019-March/068449.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>